In traditional holography and holographic image processing, the interference pattern is recorded on high-resolution film. More recently, digital computation of holograms has become possible if information about the object and the desired reconstruction angle is known. With either of these methods, however, reconstructing a viewable image from a stored hologram requires that either the film be developed, which can take an hour or more to process and dry or that the digitally computed hologram be displayed on a high-resolution electrically addressable display device which can require on the order of several minutes to compute the hologram from a collection of digitized two dimensional images. (A third technique, which is not truly holographic, employs several projectors and a rotating spiral screen to produce a real, three-dimensional image capable of being viewed on the screen from any angle.)
For years, holography has been exploited in optical pattern recognition through optical correlation. In optical correlation applications, the holographic filter must be captured optically on film or computed digitally and displayed on high-resolution devices, just as in the traditional process alluded to above with all the attendant time and processing limitations. Therefore, most of the filters computed today are not true holographic filters containing both amplitude and phase information but are phase conjugating filters that contain only phase information. The phase conjugating filters, though not containing as much information as holographic filters, are used because they are easier to compute.